Playback devices having waveguides

ABSTRACT

A playback device comprises an electroacoustic transducer; an acoustic waveguide in fluid communication with the transducer; and a housing delimiting an opening of the waveguide, the opening extending around an axis passing through the transducer. The opening may have a radial distance from the axis that varies with an azimuthal angle about the axis. An acoustic path length within the waveguide, between the transducer and the opening, is substantially constant and independent of azimuthal angle about the axis.

TECHNICAL FIELD

The disclosed technology generally relates to audio playback devices.Specifically, the disclosed technology relates to playback devicesconfigured for emitting acoustic waves with wide angular dispersion.

BACKGROUND

Options for accessing and listening to digital audio in an out-loudsetting were limited until in 2003, when SONOS, Inc. filed for one ofits first patent applications, entitled “Method for Synchronizing AudioPlayback between Multiple Networked Devices,” and began offering a mediaplayback system for sale in 2005. The Sonos Wireless HiFi System enablespeople to experience music from many sources via one or more networkedplayback devices. Through a software control application installed on asmartphone, tablet, or computer, one can play audio in any room that hasa networked playback device. Additionally, using the control device, forexample, different songs can be streamed to each room with a playbackdevice, rooms can be grouped together for synchronous playback, or thesame song can be heard in all rooms synchronously.

Given the ever growing interest in digital media, there continues to bea need to develop consumer-accessible technologies to further enhancethe listening experience.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view in an axial plane of a playbackdevice configured in accordance with embodiments of the disclosedtechnology.

FIG. 2 is a cross-sectional plan view of the playback device of FIG. 1in a plane perpendicular to the axial plane of FIG. 1.

FIG. 3A is a first cross-sectional side view of a waveguide portionalong corresponding angle around the axis of the playback device of FIG.1.

FIG. 3B is a second cross-sectional side view of a waveguide portionalong corresponding angle around the axis of the playback device of FIG.1.

FIG. 3C is a third cross-sectional side view of a waveguide portionalong corresponding angle around the axis of the playback device of FIG.1.

FIG. 4 is an isometric view of an upper surface of a waveguide.

FIG. 5A is a cross-sectional side view of a portion of a playback deviceconfigured in accordance with another embodiment of the disclosedtechnology.

FIG. 5B is a top view of an upper surface of a waveguide of the playbackdevice of FIG. 5 a.

FIG. 6 is top view of a portion of the playback device of FIG. 1.

FIG. 7 is a schematic cross-sectional view of hollow tube resonatorconfigured in accordance with embodiments of the disclosed technology.

FIG. 8 is an isometric view of a portion of a playback device configuredin accordance with another embodiment of the disclosed technology.

FIG. 9A is a cross-sectional view of a first hollow tube resonator.

FIG. 9B is a cross-sectional view of a second hollow tube resonator.

FIG. 9C is a cross-sectional view of a third hollow tube resonator.

FIG. 10A is a first polar plot of frequency components of an acousticsignal generated by an electroacoustic transducer.

FIG. 10B is a second polar plot of frequency components of an acousticsignal generated by an electroacoustic transducer.

FIG. 11 is an isometric side view of a portion of the playback device ofFIG. 1.

FIG. 12 is a plot showing angular variation of an axial dimension of awaveguide opening.

FIG. 13A is a side view of a portion of a playback device configured inaccordance with another embodiment of the disclosed technology.

FIG. 13A is a top view of an upper surface of a waveguide of theplayback device of FIG. 13A.

FIG. 14A is an isometric side view of a portion of a playback deviceconfigured in accordance with another embodiment of the disclosedtechnology.

FIG. 14B is a plan view of an upper surface of a waveguide of theplayback device of FIG. 14A.

FIG. 15A is a side view of a portion of a playback device configured inaccordance with another embodiment of the disclosed technology.

FIG. 15B is an isometric view of an upper surface of a waveguide of theplayback device of FIG. 15A.

FIG. 16A is a side view of a portion of a playback device configured inaccordance with another embodiment of the disclosed technology.

FIG. 16B is an isometric view of a waveguide of the playback device ofFIG. 16A.

FIG. 17 is a side view of a playback device configured in accordancewith another embodiment of the disclosed technology.

DETAILED DESCRIPTION

A conventional tweeter system in an audio playback device includes adiaphragm that is displaced in response to an alternating electricalsignal, thereby generating high-frequency acoustic waves (for example,acoustic waves having a frequency of between about 2 kilohertz (kHz) andabout 20 kHz). The diaphragm in many cases is shaped as a cupola, andmay be surrounded by an acoustic lens that diffracts the generatedacoustic waves. A cupola-shaped diaphragm and an acoustic lens can beused to achieve angular dispersion of the waves as they are emitted fromthe tweeter system. However, due to the relatively short wavelengths ofthe emitted waves in comparison with the aperture of the acoustic lens,the angular dispersion of the emitted waves is limited to a relativelynarrow angle. An angular dispersion of around 10 degrees from normal canbe typical for waves having frequencies in the middle of the tweeter'soperating range (e.g., between about 6 kHz and about 10 kHz).

Angular dispersion is particularly desirable for playback devicesdesigned to be used in non-reverberant environments (e.g., outdoorenvironments). In typical outdoor scenarios, for example, such aplayback device is not surrounded by walls, and therefore angulardispersion is necessary to ensure listeners at different angles aroundthe playback device are able to hear audio generated by the playbackdevice. It is desirable for users located at different angles around theplayback device to have similar listening experiences.

According to a first embodiment of the disclosed technology, a playbackdevice includes an electroacoustic transducer (e.g., a speaker driver),and an acoustic waveguide in fluid communication with the transducer. Ahousing of the playback device delimits an opening of the waveguide, theopening extending around an axis passing through the transducer andhaving a radial distance from the axis that varies with an azimuthalaxis about the axis. An acoustic path length within the waveguide,between the transducer and the opening, is substantially constant andindependent of azimuthal angle about the axis.

Providing an acoustic path length that is substantially constant andindependent of azimuthal angle causes acoustic wave fronts emitted fromthe opening to spread out evenly, resulting in substantially uniformdirectivity such that listeners positioned at different locations aroundthe playback device will have similar listening experiences.

According to a second embodiment of the present invention, a playbackdevice includes an electroacoustic transducer, and an acoustic waveguidein fluid communication with the transducer. A housing of the playbackdevice delimits an opening of the waveguide, the opening extendingaround an axis passing through the transducer. The waveguide is boundedon one side by an axial wall, and an absorber is disposed between theaxis and the axial wall and configured to attenuate acoustic waveswithin a predetermined frequency band. The absorber thereby reduces avariation of intensity around the axis of acoustic waves generated bythe transducer and emitted from the opening within the predeterminedfrequency band.

According to a third embodiment of the present invention, a playbackdevice includes an electroacoustic transducer, and an acoustic waveguidein fluid communication with the transducer. A housing of the playbackdevice delimits an opening of the waveguide, the opening extendingaround an axis passing through the transducer. The opening has adimension in a direction aligned with the axis that varies with anazimuthal angle about the axis, thereby reducing a variation ofintensity around the axis of acoustic waves generated by the transducerand emitted from the opening.

Reducing a variation of intensity around the axis of acoustic wavesgenerated by the transducer and emitted from the opening leads to moreuniform directivity, such that listeners positioned at differentlocations around the playback device will have similar listeningexperiences.

FIG. 1 is a cross-sectional side view in an axial plane of a playbackdevice configured in accordance with embodiments of the disclosedtechnology configured in accordance with embodiments of the disclosedtechnology. The playback device 100 has an upper housing 102 a and alower housing 102 b (collectively referred to as a housing 102). Anaxial wall 104 of the upper housing 102 a is received by a recess,indent or groove 105 of the lower housing 102 b, and extends in an axialdirection of an axis AB (referred to hereafter to as the axialdirection) of a plane ABCD passing through the playback device 100.

The upper housing 102 a and the lower housing 102 b form an uppersurface 106 and a lower surface 108, respectively, of an acousticwaveguide 112. The upper surface 106 has an aperture 110 for receivingan electroacoustic transducer 114. The transducer 114 is disposed on theaxis AB and includes a dome or a cupola 116 in fluid communication withthe waveguide 112. The lower surface 108 extends from the axial wall 104and includes a recess 118 configured to receive the cupola 116. Theupper surface 106 and the lower surface 108 extend toward an opening 120of the waveguide 112.

The cupola 116 is configured to be displaced in the direction of theaxis AB in response to an alternating electric signal received by thetransducer 114, thereby generating acoustic waves. In this embodiment,the transducer 114 is a tweeter, and the transducer 114 producesacoustic waves having a relatively high frequency, for example betweenabout 2 kHz and about 20 kHz A portion of the upper surface 106 of thewaveguide 112 surrounds the transducer 114, and at least partiallyaxially overlaps the transducer 114 with respect to the axis AB. Inother examples, the lower surface 108 surrounds the transducer 114.

The opening 120 of the waveguide 112 has a dimension in the axialdirection that is relatively small compared with the wavelengths ofacoustic waves generated by the transducer 114. Waves generated by thetransducer typically have wavelengths, for example, between about 2centimeters (cm) and 20 cm. The dimension of the opening 120 in theaxial direction in this embodiment is less than 1 cm, resulting in arelatively wide angular dispersion in planes coplanar with the axis AB

FIG. 2 is a cross-sectional plan view of the playback device of FIG. 1in a plane perpendicular to the axial plane of FIG. 1. FIG. 2 shows aprojection of the opening 120 of the waveguide 112 onto a plane CDEFperpendicular to the plane ABCD. In the illustrated embodiment, theopening 120 extends around the axis AB, and subtends an angle of between180 degrees and 360 degrees from the axis AB. In some embodiments, forexample, the opening 112 may subtend an angle of less than 180 degrees.Experiments have shown that angular dispersions of between 180 degreesand 240 degrees are suitable for outdoor usage, whilst reducing theimpact of back reflection in situations where the playback device 100 isplaced adjacent a wall. Back reflections from a wall may lead toundesirable acoustic effects, such as those caused by interferencebetween back-reflected waves and waves arriving directly from theplayback device. In other embodiments, however, the opening 120 subtendsa suitable angle less than 180 degrees (e.g., 90 degrees, 135 degrees,170 degrees).

The radial distance from the axis AB to the opening 120 varies withazimuthal angle about the axis AB. In this embodiment, the opening 120has a minimum radial distance from the axis AB in a direction CD, and amaximum radial distance from the axis AB in a direction EF that isperpendicular to the direction CD. The projection of the opening 120onto the plane CDEF is elongate, extending farther in the direction EFthan in the direction CD. The projection follows an arc of a stadiumhaving a straight portion GH and two circular arc portions GJ and HK. Inother examples, openings may follow other paths, for example anelliptical arc, an oval arc, or an irregular arc. In some examples, anopening may follow a complete path around an axis. In some examples, aprojection of an opening may have substantially the same extent in twoperpendicular directions. For example, a projection of an opening mayfollow a circular arc or a complete circular path.

The axial wall 104 comprises a concave portion and two convex portions.The concave portion has a projection onto the plane CDEF of a circulararc centered at the axis AB. The two convex portions have projectionsonto the plane CDEF of circular arcs centered at a point O outside theplayback device 100, the projections passing between the point O and theaxis AB. The convex portions leave space for a carrying handle at a rearside of the playback device 100, for example. The concave portionpartially surrounds the transducer 114 and maintains a constant radialseparation from a center portion of the transducer 114, through whichthe axis AB passes, which is advantageous for reducing detrimentalinterference effects, as will be described later. In this embodiment,the axial wall 104 subtends an angle of 102 degrees from the axis AB.

FIGS. 3A-3D are cross-sectional side views of a waveguide portion alongcorresponding angles around the axis of the playback device of FIG. 1.The cross-section in the plane ABCD, as shown in FIG. 3A, follows a pathhaving a first substantially S-shaped, serpentine section 300 acomprising a first local minimum 302 a, a first point of inflection 304a, and a first radial portion 306 a, the first radial portion 306 aextending toward the opening 120. The first radial portion 306 isperpendicular to the axis AB, which results in a maximum intensity ofacoustic waves being emitted in a direction perpendicular to the axisAB. It is envisaged that, during operation, the playback device 100 willoften be positioned with the axis AB in a vertical direction, and withthe opening 120 at a similar elevation to the ears of listeners. It istherefore desirable for a maximum intensity of acoustic waves to beemitted in a direction perpendicular the axis AB.

The cross-section in the plane ABLM (which has an angle of 30 degrees tothe plane ABCD, as shown in FIG. 2), is shown in FIG. 3B and follows apath having a second substantially S-shaped, serpentine section 300 bcomprising a second local minimum 302 b, a second point of inflection304 b, and a second radial portion 306 b. The second radial portion 306b extends toward the opening 120 and is perpendicular to the axis AB.The radial distance from the axis AB to the opening 120 in the ABLMcross-section is greater than the radial distance from the axis AB tothe opening 120 in the ABCD cross-section, and accordingly the radialextent x_(b) of the second S-shaped section 300 b is greater than theradial extent x_(a) of the first S-shaped section 300 a.

The axial depth of the second local minimum 302 b is less than the axialdepth of the first local minimum 302 a, and the axial separation y_(b)of the two ends of the second substantially S-shaped section 300 b isless the axial separation y_(a) of the two ends of the firstsubstantially S-shaped section 300 a. Furthermore, portions of thesecond substantially S-shaped section 300 b are less curved thancorresponding portions of the first substantially S-shaped section 300a.

The cross-section in the plane ABNP (which has an angle of 60 degreeswith respect to the plane ABCD, as shown in FIG. 2), is shown in FIG. 3Cand follows a path having a third substantially S-shaped, serpentinesection 300 c comprising a third local minimum 302 c, a third point ofinflection 304 c, and a third radial portion 306 c. The third radialportion 306 c extends toward the opening 120 and is perpendicular to theaxis AB. The radial distance from the axis AB to the opening 120 in theABNP cross-section is greater than the radial distance from the axis ABto the opening 120 in the ABLM cross-section, and accordingly the radialextent x_(c) of the third S-shaped section 300 b is greater than theradial extent x_(b) of the second S-shaped section 300 a.

The axial depth of the third local minimum 302 c is less than the axialdepth of the second local minimum 302 a, and the axial separation y_(c)between the two ends of the third substantially S-shaped section 300 cis less the axial separation y_(b) of the two ends of the secondsubstantially S-shaped section 300 b. Furthermore, portions of the thirdsubstantially S-shaped section 300 b are less curved than correspondingportions of the second substantially S-shaped section 300 a.

The cross-section of the waveguide 100 in the plane ABEF, as shown inFIG. 3D, follows a path having a straight section 300 d. Thesubstantially straight section 300 d extends toward the opening 120 andis perpendicular to the axis AB. The radial distance from the axis AB tothe opening 120 in the ABEF cross-section is greater than the radialdistance from the axis AB to the opening 120 in the ABNP cross-section,and accordingly the radial extent x_(d) of the straight section 300 d isgreater than the radial extent x_(c) of the third S-shaped section 300a.

Although the radial distance from the axis AB to the opening 120 isdifferent in each of the cross-sections of FIGS. 3A-3D, the varyingcurvature and axial variation of the waveguide, as described above,result in an acoustic path length within the waveguide 112, between thetransducer 114 and the opening 120, that is substantially the same(e.g., within about 1%, within about 2%, within about 5%, within about10%) for each of the cross-sections. Moreover, an acoustic path lengthwithin the waveguide 112, between the transducer 114 and the opening120, is substantially constant and independent of azimuthal angle aboutthe axis AB. In this embodiment, the acoustic path length is constantbetween the center portion of the transducer 114, through which the axisAB passes, and the opening 120.

FIG. 4 is an isometric view of an upper surface of a waveguide. FIG. 4shows a contour corresponding to the upper surface 106 of the waveguide100, omitting the axial wall 104 for clarity. As described above, theupper surface 106 has an aperture 110 for receiving the transducer 114.The upper surface 106 is smoothly contoured so that as the radialdistance to the axis AB varies, the curvature and axial extent of theupper surface 106 vary to compensate the variation in radial distance,such that the acoustic path length from the transducer 114 to theopening 120 remains substantially constant. Each of the curves drawn onthe surface 106 has an equal length and represents an acoustic path fromthe transducer 114 to the opening 120. The length of each curve in thisembodiment is a predetermined length, corresponding to the radial extentx_(d) of the straight section in FIG. 3D. In some embodiments, forexample, the length is between about 40 mm and about 70 mm, betweenabout 55 mm and about 65 mm, or approximately 63 mm. In other examplesthe acoustic path length may be different, for example, depending on therelative size of the housing and transducer.

The contouring of the waveguide in the illustrated embodiment has beenselected to minimize sharp variations or areas of high curvature insidethe waveguide, whilst maintaining an acoustic path length between thetransducer and the opening that is substantially constant andindependent of azimuthal angle about the axis AB. As those of ordinaryskill in the art will appreciate, sharp variations inside a waveguidemay lead to undesirable acoustic effects such as internal reflectionsand dispersion. For each cross-section coplanar with the axis AB, theradial extent of the substantially S-shaped section is predetermined bythe radial distance between the axis AB and the opening 120, which isturn is substantially predetermined by the stadium shape of the playbackdevice 100. In this embodiment, the curve of the S-shaped section ineach cross-section is defined by four control points, as shown in FIGS.3A, 3B, and 3C, and a curve-fitting algorithm is used to determine asmoothest curve between the four control points, subject to theconstraint that the curve for each cross-section must be perpendicularto the axis at both ends of the S-shaped section. In this embodiment,for each cross-section, the relative Cartesian positions of the controlpoints with respect to a first control point at the transducer end ofthe S-shaped section are given by (0, 0), (x/4, −y/5), (x,y), (5x/6,1.1y), where y is the radial extent of the S-shaped cross-section aspredetermined by the dimensions of the playback device 100, and where xis varied such that the length of the curve is given by y_(d), where inthis embodiment y_(d)=63.45 mm.

Providing an equal acoustic path length from the transducer 114 to theopening 120 can result in waves generated by the transducer 114 thatreach the opening 120 with a phase that is substantially constant andindependent of azimuthal angle about the axis. Acoustic wave frontspropagating from the opening 120 can therefore spread out more evenlythan conventional waveguides with varying path-lengths, resulting insubstantially uniform directivity in which listeners positioned atdifferent locations around the playback device 100 will have similarlistening experiences. By contrast, if the acoustic path length withinthe waveguide was not substantially constant, wave fronts wouldpropagate from the opening 120 at frequency-dependent angles,potentially resulting in non-uniform, frequency-dependent directivity.In particular, frequency-dependent directivity may result infrequency-dependent regions of destructive and constructiveinterference, such that listeners at different locations may havedifferent listening experiences, even if the listeners are positioned atsubstantially the same distance away from the playback device 100.

Other examples are envisaged in which an acoustic path length within awaveguide is substantially constant. In some examples, the axialseparation of two ends of a waveguide section is substantially constantand independent on azimuthal angle, and a variation in radial distanceis compensated by varying the curvature of one or more portions of awaveguide. FIG. 5A is a cross-sectional side view of a portion of aplayback device configured in accordance with another embodiment of thedisclosed technology. FIG. 5A shows a cross-section of a playback device500 in a plane coplanar with an axis A′B′. The playback device 500 has astadium-shaped cross-section in a plane perpendicular to the axis A′B′,and has an upper housing 502 a and a lower housing 502 b, which delimita waveguide 504 having an opening 506 that extends around the axis A′B′.In this embodiment, the position of the opening is constant andindependent on azimuthal angle about the axis A′B′. In order tocompensate a varying radial distance between the axis A′B′ and theopening 506, the waveguide is contoured to include two local maxima 508a and 508 b. FIG. 5B is a top view of an upper surface of a waveguide ofthe playback device of FIG. 5A. FIG. 5B shows the upper housing 502 a ofthe playback device 500.

In other examples, a variation in radial distance is compensated byvarying the axial extent of a waveguide, without substantially varyingthe curvature of any portion of the waveguide. In some examples, aplayback device further includes a low-frequency electroacoustictransducer such as a woofer. In such examples, it may be desirable tolimit the axial separation between the opening of the waveguide and thelow-frequency transducer, thereby limiting a separation of apparentsources of acoustic waves of different frequencies, which may otherwisegive rise to an undesirable experience for listeners. In such examples,varying the curvature of one or more portions of the waveguide, forexample by including local extrema in the waveguide, may provide asuitable means of compensating for a variation in radial distance.Suitable means for compensating a varying radial distance may depend onthe geometry of the playback device.

In the embodiment described above, a proportion of the acoustic wavesgenerated by the transducer 114 is reflected by the axial wall 104. Thereflected acoustic waves can propagate through the waveguide 112 and areemitted from the opening 120 along with waves propagating directly fromthe transducer 114 toward the opening 120. Interference between thereflected waves and the direct waves may result in regions outside theplayback device 100 in which the intensities of acoustic waves ofparticular frequencies are reduced and/or increased with respect to thedirect waves alone. With reference to FIG. 2, for instance, adestructive interference region may arise between the dashed lines 200 aand 200 b, which corresponds to the region into which waves reflectedfrom the axial wall 104 are emitted. As described above, the concaveportion of the axial wall 104 partially surrounds the transducer 114 andmaintains a constant radial separation from the center portion of thetransducer 114. Acoustic waves reflected from the axial wall 104therefore propagate in parallel with acoustic waves propagating directlyfrom the transducer 114, potentially causing an intensity reduction ofacoustic waves of particular frequencies throughout the destructiveinterference region 200.

Destructive interference between reflected waves and direct waves occurswhen the reflected waves propagate in antiphase with the direct waves,such the phase difference between the direct waves and the reflectedwaves is π radians or 180°. In this embodiment, destructive interferenceoccurs when acoustic waves generated by the transducer 114 have awavelength that is approximately four times an acoustic path length fromthe center portion of the transducer 114 and the axial wall 104. Afterbeing reflected by the axial wall 104, acoustic waves of this wavelengthare in antiphase with direct waves generated by the transducer 114 andtherefore destructive interference occurs within the destructiveinterference region.

In practice, acoustic waves are not generated at a single point, but aregenerated throughout the central portion of the transducer (accordingly,over a central region of the cupola), leading to a frequency band overwhich destructive interference occurs. The frequency band contains apeak destructive interference frequency at which maximum destructiveinterference occurs, and extends to frequencies above and below the peakdestructive interference frequency. Destructive interference furtheroccurs at higher frequency bands containing odd multiples of the peakdestructive interference frequency (for example, three times the peakdestructive interference frequency and five times the peak destructiveinterference frequency). In this embodiment, most of these higherfrequency bands are beyond the range of operation of the transducer 114,and therefore effects on acoustic waves in these bands have negligibleeffect on listener experience.

FIG. 6 is top view of a portion of the playback device of FIG. 1. FIG. 6shows a portion of the lower housing 102 b of the playback device 100(FIG. 1), including the indent 105 for receiving the axial wall 104 ofthe upper housing 102 a. As described above, a portion of the lowerhousing 102 b forms the lower surface 108 of the waveguide 110, whichcontains the recess 118 for receiving the cupola 116 of the transducer114. The lower surface 108 delimits an opening to a hollow tuberesonator or attenuator 600 between the recess 118 and the indent 105.The hollow tube resonator 600 is thereby disposed between the axis ABand the axial wall 104. FIG. 6 further shows four axial pillars 602 a,602 b, 602 c, and 602 d, collectively referred to as axial pillars 602,disposed within the waveguide 112. The axial pillars 602 support theupper housing 102 a, and each of the axial pillars 602 is threaded toreceive a screw in order to secure the upper housing 102 a to the lowerhousing 102 b. Other methods of fastening such as, for instance,bonding, snap-fit, and/or friction-fit may also be used in otherexamples.

The resonator 600 is configured to attenuate acoustic waves within apredetermined frequency band. In this embodiment, the predeterminedfrequency band corresponds to a frequency band in which destructiveinterference occurs between reflected and direct waves, as describedabove. When acoustic waves pass over one of the resonator 600 within thepredetermined frequency band of the resonator, air within the hollowtube will resonate. The resulting resonance is substantially inantiphase with the acoustic waves passing over the resonator, and causespartial cancellation of oscillations of a pressure field caused by theacoustic wave. The resonator 600 thereby acts as an absorber of acousticwave energy. As a result of the cancellation, oscillations of theresultant pressure field are attenuated, causing attenuation of theacoustic waves reflected by the axial wall 104 and propagating towardthe opening 120. Accordingly, the effect of destructive interference ofwaves within the predetermined frequency band of the resonator isreduced, and a variation of intensity around the axis AB of acousticwaves generated by the transducer 114 and emitted from the opening 120,within the predetermined frequency band of the resonator, is reduced.

The predetermined frequency band for the resonator 600 overlaps with afrequency band in which destructive interference occurs. As explainedabove, destructive interference occurs within a frequency band thatcontains a peak destructive interference frequency and extends tofrequencies above and below the peak destructive interference frequency.Similarly, the predetermined frequency band of a resonator has a peakresonant frequency and extends to frequencies above and below the peakresonant frequency. A peak resonant frequency of a resonator correspondsto a maximum attenuation frequency, at which attenuation of wavespassing over the resonator is maximum. The peak destructive interferencefrequency in this embodiment approximately corresponds to that ofacoustic waves with a wavelength that is four times an acoustic pathlength from the center portion of the transducer 114 and the axial wall104. In this embodiment, the predetermined frequency band for theresonator 600 includes the frequency corresponding to acoustic waveswith a wavelength that is four times an acoustic path length from thecenter portion of the transducer 114 and the axial wall 104.

FIG. 7 is a schematic cross-sectional view of hollow tube resonatorconfigured in accordance with embodiments of the disclosed technology.FIG. 7 shows a cross-section of the hollow tube resonator 600 in a planeparallel to the axis AB. The dashed line 701 in FIG. 7 is parallel tothe axis AB. The resonator 600 has a substantially flat circular lowersurface 702 disposed at an axial depth d from an open end 704. Theresonator 600 further has a conical curved surface 706 inclined at adraft angle θ to the axis AB. In this embodiment, θ is approximately 1.5degrees, the draft depicted in FIG. 7 is shown larger then 1.5 degreesfor clarity. The resonator 600 therefore has the shape of a frustum, ofwhich the cross-sectional area in planes perpendicular to the axis ABdecreases with distance from the open end 704. Providing a non-zerodraft angle, such that the cross-sectional area decreases with distancefrom the open end 704, assists in the manufacture of the lower housing102 b. For example, the non-zero draft angle may facilitate the removalof a mold when the portion of the lower housing forming the resonator ismanufactured by a molding process. In some examples, a draft angle ofzero may be used. In other examples, a suitable draft angle of greateror less than 1.50 may be used. In some instances, varying the draftangle can have an effect on the frequency band of a resonator, and itmay be beneficial to keep the draft angle relatively small, for exampleless than 10 degrees or less than 5 degrees.

Due to the relatively small draft angle θ, a first peak resonantfrequency approximately corresponds to acoustic waves having awavelength four times the axial depth of the resonator 600. For a hollowtube resonator, further peak resonant frequencies occur approximately atodd multiples of the first peak resonant frequency. In the presentembodiment, the predetermined frequency band of the resonator 600overlaps with a frequency band within which destructive interferenceoccurs, which including a peak destructive interference frequency.Higher resonant frequency bands of the resonator 600 also overlap withhigher frequency bands containing odd multiples of the peak destructiveinterference frequency. As discussed above, destructive interference mayalso occur within these higher frequency bands. The resonator 600 istherefore also configured to reduce destructive interference in thesehigher frequency bands, to the extent that they correspond tofrequencies generated by the transducer. In other examples the absorbermay be configured to reduce signals at the peak destructive interferencefrequency without considering other frequencies.

FIG. 8 is an isometric view of a portion of a playback device configuredin accordance with another embodiment of the disclosed technology. FIG.8 shows a portion of the lower housing 802 b of a playback device. Insome embodiments, for instance, the playback device has the same orsimilar construction as the playback device 100 (FIG. 1), but with adifferent configuration of absorbers, as will be described hereafter.The lower housing 802 b defines an indent 805 for receiving an axialwall of an upper housing. A portion of the lower housing 802 forms alower surface 808 of a waveguide, which contains a recess 818 forreceiving a cupola of an electroacoustic transducer. The lower surface808 is indented between the recess 818 and the indent 805 to form hollowtube resonators, including hollow tube resonators 800 a, 800 b, and 800c. The resonators 800 a, 800 b, and 800 c are thereby disposed betweenthe axis AB and the axial wall 104. The playback device 100′ has fivehollow tube resonators in total.

Each of the resonators 800 a, 800 b, and 800 c is configured toattenuate acoustic waves within a respective predetermined frequencyband. In this embodiment, the respective predetermined frequency bandfor each of the resonators corresponds to a frequency band in whichdestructive interference occurs between reflected and direct waves, asdescribed above.

In the present embodiment, the predetermined frequency band for each ofthe resonators 800 a, 800 b, and 800 c overlaps with a frequency band inwhich destructive interference occurs. The predetermined frequency bandfor each of the resonators 800 a, 800 b, and 800 c includes thefrequency corresponding to acoustic waves with a wavelength that is fourtimes an acoustic path length from the center portion of the transducerand the axial wall.

FIG. 9A are cross-sectional views of three corresponding hollow tuberesonators 800 a, 800 b, and 800 c, in a plane parallel to the axis AB.The dashed lines in FIGS. 9A, 9B, and 9C are parallel to the axis AB.Resonator 800 a has a shape similar to that of the resonator 600 shownin FIG. 7, but has an axial depth of d_(a) and a draft angle of θ_(a).

Resonator 800 b and hollow tube resonator 800 c have shapes similar tothat of resonator 800 a, the shapes being frustums with draft angles ofθ_(b) and θ_(c), respectively. In this embodiment, θ_(a), θ_(b), andθ_(c), are all approximately equal to 1.5 degrees, but they may bedifferent in other examples. The axial depth of resonator 800 b is d_(b)and the axial depth of resonator 800 c is d_(c). In this embodiment, theaxial depth de of resonator 800 c is equal to the axial depth d_(a) ofresonator 800 a. The axial depth d_(b) of resonator 800 b is greaterthan the axial depth d_(a) of resonator 800 a. The predeterminedfrequency band for resonator 800 b is lower than the predeterminedfrequency band for resonator 800 a and resonator 800 c. In particular,the lowest peak resonant frequency of resonator 800 b is lower than thelowest peak resonant frequency of resonators 800 a and 800 c.Accordingly, the maximum attenuation frequency in the predeterminedfrequency band of resonator 800 b is lower than the maximum attenuationfrequency in the predetermined frequency band of resonators 800 a and800 c. The resonators are configured such that a target frequency isincluded in a range between the maximum attenuation frequency ofresonator 800 b and the maximum attenuation frequency of resonators 800a and 800 c. In this embodiment, the target frequency corresponds toacoustic waves having a wavelength that is four times an acoustic pathlength between the center portion of the transducer and the axial wall.As discussed above, this target frequency is approximately equal to thepeak destructive interference frequency. Having at least one absorberconfigured to attenuate acoustic waves at a frequency below a targetfrequency, and at least one absorber configured to attenuate waves at afrequency above a target frequency, where the target frequency isapproximately equal to the peak destructive interference frequency,allows for attenuation of acoustic waves over a large proportion of thefrequency band within which destructive interference occurs.

FIGS. 10A and 10B are corresponding polar plots of frequency componentsof an acoustic signal generated by an electroacoustic transducer. FIGS.10A and 10B show experimental results demonstrating the effect ofdisposing absorbers between a central axis and an axial wall of aplayback device as described above. Each of the curves in FIGS. 10A and10B corresponds to acoustic waves of a particular frequency, the radialdistance of the curve from the origin representing the intensity of theacoustic waves of that frequency. To generate the curves, a tone ofparticular frequency is emitted by the transducer and the receivedintensity at various azimuth angles is measured. The process is thenrepeated at different frequencies. FIG. 10A corresponds to an experimentin which no absorbers were included in the waveguide. A significantreduction in the intensity of waves within a frequency band is observedin a destructive interference region. In this embodiment, a peakdestructive interference frequency of around 7 kHz is observed whichdepends on the particular dimensions of the device under test. FIG. 10Bcorresponds to an experiment in which three hollow tube resonators aredisposed between the central axis and the axial wall. In thisembodiment, a first resonator having a maximum attenuation frequency ofaround 6.8 kHz is disposed azimuthally between a second and a thirdresonator each having a maximum attenuation frequency of around 7.2 kHz.The intensity of waves in the frequency band is increased in thedestructive interference region compared with FIG. 10A, demonstratingthat the effect of destructive interference has been reduced. Thevariation of intensity around the central axis of the acoustic waveswithin the frequency band is thereby reduced by the inclusion ofabsorbers. Providing a more uniform directivity may result in animproved user perception of sound quality across a wider area.

The resonators shown in FIG. 8 are disposed the same distance from thecenter portion of the transducer 114, and are evenly spaced around thecenter portion of the transducer 114. In other words, the resonators mayhave a substantially equal radial spacing about the center portion ofthe transducer. In this embodiment, spacing the absorbers evenlyminimizes directional variation in the attenuation of acoustic waves. Inother examples, resonators may not be disposed the same distance from acenter portion of a transducer, or may not be evenly spaced around acenter portion of a transducer. This may particularly be the case inexamples where an axial wall does not include a circular concavesection. For example, multiple resonators may be disposed at differentdistances from a center portion of a transducer. This may allow moreresonators to be included, which may allow for the attenuation ofacoustic waves having a greater range of frequencies. In some examples,different numbers of resonators are used, for example three resonators.

In some examples, different types of absorbers are used instead of theresonators described above. For example, closed hollow tube resonatorsmay have different shapes to the resonators described above. Closedhollow tube resonators may have circular cross-sections of differentsizes, or may have non-circular cross-sections. Varying the shape of ahollow tube resonator affects the peak resonant frequencies of theresonator, and also the sharpness of the associated resonance peaks.This further affects the width of the frequency bands over whichattenuation of acoustic waves may be achieved. In other examples, hollowtube resonators may be used that are open at both ends. In contrast withclosed hollow tube resonators such as those described above, open hollowtube resonators, which are open at both ends, have peak resonantfrequencies that are approximately even multiples of a lowest peakresonant frequency. In the absence of absorbers, it is expected thatconstructive interference will occur at higher frequency bands includingeven multiples of the peak destructive interference frequency. Usingopen hollow tube resonators to attenuate acoustic waves in a frequencyband including the peak destructive interference frequency may thereforereduce the effects of constructive interference in these higherfrequency bands.

In some examples, acoustic dampers may be used as absorbers, for examplefoam acoustic dampers. Acoustic damping may attenuate acoustic wavesover a broader frequency range than resonators, but may have otherdetrimental effects such as those resulting from absorption of acousticwaves in frequency bands not prone to destructive interference.

Different numbers of absorbers may be used in certain embodiments. Forexample, a single absorber may be simpler to manufacture than more thanone absorber, but may not provide as effective performance. The numberof absorbers included in a particular example may balance the complexityof manufacture against the uniformity of response achieved.

In some embodiments, an opening of a waveguide has an axial dimensionthat varies with an azimuthal angle about an axis. FIG. 11 is anisometric side view of a portion of the playback device of FIG. 1. Asshown, the opening 120 has a dimension in a direction aligned with theaxis AB that varies with an azimuthal angle about the axis AB. Varyingthe dimension of the opening 120 with azimuthal angle about the axis ABcan reduce a variation of intensity around the axis AB of acoustic wavesgenerated by the transducer 114 and emitted from the opening 120. Avariation of intensity around the axis AB may be caused by thenon-circular geometry of the playback device (as described above, aprojection of the opening 120 onto a plane perpendicular to the axis ABfollows an arc of a stadium). A variation of intensity around the axisAB may also be caused by other features of the geometry of the playbackdevice 100, for example the axial pillars 602.

In the playback device 100, experiments were performed to determine asuitable variation of the axial dimension of the opening 120 withazimuthal angle. Table 1 below shows the resulting axial dimension ofthe opening 120 at different angles from CD. The opening heights havereflective symmetry about the plane ABCD due to the symmetry of theplayback device about this plane. The maximum angle from CD in thisembodiment is 129 degrees, corresponding to the angle at which the axialwall 104 interrupts the opening 120. FIG. 12 shows a plot of thevariation of the axial dimension of the opening 120 with azimuthal anglefrom CD. The opening height was determined by measuring the intensityacross the frequency range at each azimuth angle by testing a waveguidewith the same path lengths and internal construction but with aconstant, predetermined axial dimension. The intensity measurements maybe made substantially tangential to the waveguide opening. From themeasured intensity a desired gain or reduction to provide substantiallyuniform intensity across all azimuth angles was determined. For example,a larger axial dimension will tend to provide a more narrow beam in thevertical dimension than a smaller axial opening because of diffraction.

Variation in the axial dimension (the opening height) provides controlof intensity across the frequency range. A larger opening will increasethe intensity of the sound reaching a listener at that angle while asmaller opening will decrease the intensity of the sound reaching thelistener. The adjustment in intensity at a particular azimuth anglerelative to the sound pressure level (SPL) at a reference 0 degree angleis determined by the relation: SPL=10 log₁₀(w/w_(ref)), where w is theaxial dimension of the slit at the particular azimuth dimension andw_(ref) is the axial dimension at zero degrees. Applying this relationto the required changes in SPL at different angles from line CD (whichis w_(ref)) leads to the following example values for the axialdimension, set out in Table 1. At the same time, the maximum axialdimension may be kept relatively small relative to the smallestwavelengths of sound in the waveguide to reduce the effect of anybeam-forming in the vertical direction. As can be seen in Table 1 below,in the embodiment of FIG. 1, the maximum axial dimension is 9.86 mm,which is smaller than the wavelength (around 17 mm) of a 20 kHzsoundwave in air, for example.

TABLE 1 Angle from CD (degrees) Axial dimension/mm 0 1.62 5 1.72 10 2.0215 2.56 20 3.36 25 4.44 30 5.75 35 7.08 40 8.19 45 8.82 50 9.08 55 9.2360 9.54 65 9.86 70 9.63 75 8.69 80 7.47 85 6.26 90 5.19 95 4.44 100 3.95105 3.70 110 3.47 115 2.80 120 1.87 125 1.22 129 1.01

FIGS. 13A, 13B, 14A, 14B, 15A, 15B, 16A, 16B, and 17 show furtherembodiments of playback devices in which an axial dimension of anopening varies with azimuthal angle. In each of these embodiments, anangular variation of intensity of waves generated by the playback devicemay result from the geometry of the playback device. The variation ofthe axial dimension of the opening reduces this variation of intensity.

FIG. 13A is a side view of a portion of a playback device configured inaccordance with another embodiment of the disclosed technology. FIG. 13Ashows a playback device 1300 in which an opening 1302 varies withazimuthal angle about an axis PR. The playback device 1300 has asubstantially stadium-shaped cross-section in a plane perpendicular tothe axis PR. FIG. 13B is a top view of an upper surface of a waveguideof the playback device of FIG. 13A. A waveguide surface 1304 of theplayback device 1300 includes radially-extending fins that createsubstantially radial waveguide channels.

FIG. 14A is an isometric side view of a portion of a playback deviceconfigured in accordance with another embodiment of the disclosedtechnology. FIG. 14A shows a playback device 1400 in which an opening1402 varies with azimuthal angle about an axis ST. FIG. 14B is a planview of an upper surface of a waveguide of the playback device of FIG.14A. An electroacoustic transducer 1404 is disposed off-center such thata radial distance r₁ from the transducer 1404 to a front portion of theplayback device 1400 is substantially the same as a radial distance r₂to a side portion of the playback device 1400. In this embodiment, axialpillars including axial pillar 1406 are disposed within a waveguide1408. The transducer 1404 is disposed within a lower surface 1410 of thewaveguide 1408.

FIG. 15A is a side view of a portion of a playback device configured inaccordance with another embodiment of the disclosed technology. FIG. 15Ashows a playback device 1500 in which an opening 1502 varies withazimuthal angle about an axis UV. As shown in FIG. 15B is an isometricview of an upper surface of a waveguide of the playback device of FIG.15A. The playback device 1500 has a circular lower waveguide surface1504 extending to the opening 1502, such that a projection of theopening 1502 onto a plane perpendicular to the axis UV is circular.Providing a circular projection ensures that an acoustic path lengthbetween the transducer 1506 and the opening 1502 is substantiallyconstant and independent of azimuthal angle about the axis UV. Sloped,chamfered portions, including chamfered portion 1508, extend fromcircular lower waveguide surface and a corresponding circular upperwaveguide surface. The playback device 1500 has a substantiallystadium-shaped cross-section in a plane perpendicular to the axis UV.

FIG. 16A is a side view of a portion of a playback device configured inaccordance with another embodiment of the disclosed technology. FIG. 16Ashows a playback device 1600 in which an opening 1602 varies withazimuthal angle about an axis WX. In this embodiment, a transducer 1604is disposed such that in response to a received electrical signal, adiaphragm 1606 (which is shaped as a cupola in this embodiment) isdisplaced in a front-facing direction of the playback device 1600 thatis perpendicular to the axis WX. Sloped, chamfered portions extend froma circular lower waveguide surface and a corresponding circular upperwaveguide surface. The playback device 1600 has a substantiallystadium-shaped cross-section in a plane perpendicular to the axis WX.FIG. 16B is an isometric view of a waveguide of the playback device ofFIG. 16A.

FIG. 17 is a side view of a playback device configured in accordancewith another embodiment of the disclosed technology. FIG. 17 shows aplayback device 1700 in which an opening 1702 varies with azimuthalangle about an axis YZ. The playback device 1700 has a substantiallycircular cross-section in a plane perpendicular to the axis YZ. Axialpillars including axial pillar 1704 are disposed within a waveguide1706.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. For example, embodiments may use a constant acoustic pathlength, varying axial dimension of the waveguide and one or moreabsorbers alone or in any combination thereof. Each of these featurescontributes to improving the uniformity of the directivity of a widedispersion waveguide. It will also be appreciated that strict adherenceto the dimensions and examples given herein is not required, these willvary with the dimensions of a playback device. Furthermore, equivalentsand modifications not described above may also be employed withoutdeparting from the scope of the invention, which is defined in theaccompanying claims.

What is claimed is:
 1. A playback device comprising: an electroacoustictransducer; an acoustic waveguide in fluid communication with thetransducer; and a housing delimiting an opening of the waveguide, theopening extending around an axis passing through the transducer andhaving a radial distance from the axis that varies with an azimuthalangle about the axis, wherein an acoustic path length within thewaveguide, between the transducer and the opening, is substantiallyconstant and independent of azimuthal angle about the axis.
 2. Theplayback device of claim 1, wherein a projection of the opening onto aplane perpendicular to the axis extends farther in a first direction inthe plane than in a second direction in the plane, the second directionbeing perpendicular to the first direction.
 3. The playback device ofclaim 1, wherein a portion of the waveguide has a substantiallyserpentine cross-section in a plane coplanar with the axis.
 4. Theplayback device of claim 2, wherein a portion of the waveguide has: afirst substantially serpentine cross-section in a first plane coplanarwith the axis; and a second substantially serpentine cross-section in asecond plane coplanar with the axis, wherein a section of the firstsubstantially serpentine cross-section has a greater curvature than acorresponding section of the second substantially serpentinecross-section.
 5. The playback device of claim 1, wherein a portion ofthe waveguide extending toward the opening is perpendicular to the axis.6. The playback device of claim 1, wherein the electroacoustictransducer comprises a diaphragm configured to be displaced in adirection substantially aligned with the axis in response to a receivedelectrical signal.
 7. The playback device of claim 1, wherein theopening has a dimension in a direction aligned with the axis that varieswith azimuthal angle about the axis, thereby reducing a variation ofintensity around the axis of acoustic waves generated by the transducerand emitted from the opening.
 8. The playback device of claim 1, whereinthe waveguide is bounded on one side by an axial wall, the playbackdevice further comprising: an absorber disposed between the axis and theaxial wall and configured to attenuate acoustic waves within apredetermined frequency band, thereby reducing a variation of intensityaround the axis of acoustic waves generated by the transducer andemitted from the opening within the predetermined frequency band.
 9. Aplayback device comprising: an electroacoustic transducer; an acousticwaveguide in fluid communication with the transducer; a housingdelimiting an opening of the waveguide, the opening extending around anaxis passing through the transducer and having a radial distance fromthe axis that varies with an azimuthal angle about the axis, wherein thewaveguide is bounded on one side by an axial wall; and an absorberdisposed between the axis and the axial wall and configured to attenuateacoustic waves within a predetermined frequency band, thereby reducing avariation of intensity around the axis of acoustic waves generated bythe transducer and emitted from the opening within the predeterminedfrequency band.
 10. The playback device of claim 9, wherein the absorbercomprises a hollow tube resonator.
 11. The playback device of claim 10,wherein a cross-sectional area of the hollow tube resonator decreaseswith distance from an open end of the hollow tube resonator.
 12. Theplayback device of claim 9, wherein the transducer has a center portionsubstantially aligned with this axis, and wherein the predeterminedfrequency band includes a frequency corresponding to acoustic waveshaving a wavelength that is four times an acoustic path length betweenthe center portion of the transducer and the axial wall.
 13. Theplayback device of claim 9, wherein the absorber is a first absorber,further comprising a plurality of absorbers disposed between the axisand the axial wall, each of the plurality of absorbers being configuredto attenuate acoustic waves within a respective predetermined frequencyband.
 14. The playback device of claim 13, wherein the plurality ofabsorbers are disposed substantially the same distance from the centerportion of the transducer.
 15. The playback device of claim 13, whereinthe plurality of absorbers are substantially evenly spaced about thecenter portion of the transducer.
 16. The playback device of claim 13,wherein: at least one of the plurality of absorbers is configured toattenuate acoustic waves within a first predetermined frequency band;and at least one of the plurality of absorbers is configured toattenuate acoustic waves within a second predetermined frequency band,the second predetermined frequency band being different from the firstpredetermined frequency band.
 17. The playback device of claim 16,wherein a range between a maximum attenuation frequency in the firstpredetermined frequency band and a maximum attenuation frequency in thesecond predetermined frequency band includes a target frequency.
 18. Theplayback device of claim 17, wherein the target frequency corresponds toacoustic waves having a wavelength that is four times an acoustic pathlength between the center portion of the transducer and the axial wall.19. The playback device of claim 9, wherein the opening has a dimensionin a direction aligned with the axis that varies with azimuthal angleabout the axis, thereby reducing a variation of intensity around theaxis of acoustic waves generated by the transducer and emitted from theopening.
 20. A playback device comprising: an electroacoustictransducer; an acoustic waveguide in fluid communication with thetransducer; and a housing delimiting an opening of the waveguide, theopening extending around an axis passing through the transducer, whereinthe opening has a dimension in a direction aligned with the axis thatvaries with an azimuthal angle about the axis, thereby reducing avariation of intensity around the axis of acoustic waves generated bythe transducer and emitted from the opening.